JP4000616B2 - Gear having excellent pitting resistance and method for producing the same - Google Patents

Description

【００６４】
実施形態例３
本例は，表２に示すごとく，実施形態例１において用いたＡ鋼の他に８種類（合計９種類）の供試鋼を準備し，肌焼鋼の組成，浸炭異常層の深さ等がピッチング強度等にどのように影響するかを定量的に評価した。
まず，準備した供試鋼の化学成分の一覧を表２に示す。
【００６５】
【表２】

【００６６】
次に，各供試鋼は，溶解，鍛伸鍛造，焼ならしを行った後に，後述する各試験用の試験片に加工し，浸炭処理行った。浸炭処理は，後述の表３に示すごとく，浸炭層のＣ濃度を０．６５％以上に保ちつつ，浸炭異常層の最大深さ及び占有面積率を変化させるように行った。具体的には，通常のガス浸炭を行ったものについては，実施形態例１の浸炭方法（図３）を基本とし，温度，時間，Ｃ．Ｐの各条件を微調整することにより行った。また，浸炭異常層を全く生成させないものについては，真空ガス浸炭処理により行った。
【００６７】
次に，歯車における歯面強度の評価するための代替試験として，図６，図７に示すごとく，ローラピッチング試験を行った。
ローラピッチング試験は，図６，図７に示すごとく，中央部分に試験部分４１１を有する小ローラ４１と，円盤状の大ローラ４２とを用いて行う。これら小ローラ４１及び大ローラ４２は，同一鋼種を用いて同じ製造工程及び浸炭処理を施して作製する。
【００６８】
小ローラ４１は，図７（ａ）に示すごとく，全長Ｌが１３０ｍｍの中央部分に，幅Ｗが２８ｍｍ，外径Ｄ₁ が２６ｍｍの試験部分４１１を設けてなる。また，大ローラ４２は，図７（ｂ）に示すごとく，厚みＴが１８ｍｍ，外径Ｄ₂ が１３０ｍｍの円盤であって，その端部４２１は図７（ｃ）に示すごとく，クラウニング半径３００ｍｍの曲面にしてある。
【００６９】
そして，図６に示すごとく，軸４２９にセットした大ローラ４２の端部４２１と小ローラ４１の試験部分４１１とを当接させた状態で，それぞれの周速に差をつけて回転させる。具体的には，小ローラ４１の回転数を２０００ｒｐｍとし，すべり率（周速差）を−４０％とした。また，潤滑は油温１２０℃のＡＴＦ（オートマチックトランスミッションフルード）により行った。
【００７０】
そして，小ローラ４１と大ローラ４２との間に一定の面圧をかけた状態で回転させてピッチング発生までの小ローラ４１の総回転数を求める。これを面圧を代えて繰り返し行う。なお，ローラピッチング試験機としては，コマツエンジニアリング（株）製のものを用いた。
【００７１】
そして，本例においては，小ローラ４１を１０⁷ 回以上回転させてもピッチングが発生しない面圧（１０⁷ 回耐久面圧）をピッチング強度（ＭＰａ）として評価に用いた。
試験結果を表３に示す。
【００７２】
表３より知られるごとく，浸炭異常層の最大深さが０のもの及び４０μｍを超えるもの（試料Ｎｏ．１，６，１２）については，すべてピッチング強度が１８００ＭＰａ以下と低い値になった。
また，浸炭異常層の最大深さが５〜４０μｍの範囲内のものであっても，浸炭異常層の占有面積率が７０％未満の場合（試料Ｎｏ．３，１４）には，同じく１８００ＭＰａ以下という低い値となった。逆に，浸炭異常層の占有面積率が７０％以上の場合でも，上記のごとく浸炭異常層の最大深さが４０μｍを越える場合（試料Ｎｏ．６）には，同じく１８００ＭＰａ以下という低い値となった。
【００７３】
また，浸炭異常層の最大深さ及び占有面積率が良好であっても，鋼の成分範囲が本発明範囲外の場合（試料Ｎｏ．１３），及び浸炭異常層の最大深さ及び占有面積率が良好であり，かつ，鋼の成分範囲が本発明範囲内にあっても，上述した関係式の値が１．５未満の場合（試料Ｎｏ．１０）については，ピッチング強度が低い結果となった。
【００７４】
次に，歯車の歯元強度を推定するための代替試験として，小野式回転曲げ疲労試験を行った。
小野式回転曲げ疲労試験は，図８に示すごとき形状の試験片５を用い，ＪＩＳ−Ｚ２２７４の「金属材料の回転曲げ疲れ試験方法」に準じて行った。なお，回転数は３６００ｒｐｍとした。
そして，本例においては，１０⁷ 回以上回転させても破断しない曲げ応力（１０⁷ 回耐久応力）を評価に用いた。
【００７５】
試験結果を表３に示す。
表３により知られるごとく，試料Ｎｏ．６以外のものは５２Ｋｇｆ／ｍｍ² 以上となり良好な結果が得られた。これに対し，試料Ｎｏ．６は，小野式回転曲げ強度が最も低い値となった。これは，浸炭異常層の最大深さが深すぎたためであると考えられる。
【００７６】
【表３】

【００７７】
以上の試験結果から，鋼の成分範囲，関係式の具備，浸炭異常層の適度な形成によって，ピッチング強度を確実に向上させることができることがわかる。また，ピッチング強度を向上できる範囲においては，小野式回転曲げ強度も良好であり，歯車における歯元強度も良好に維持することができることがわかる。
【００７８】
【発明の効果】
上述のごとく，本発明によれば，低コストで製造することができ，かつ，耐ピッチング性に優れた歯車及びその製造方法を提供することができる。
【図面の簡単な説明】
【図１】実施形態例１における，浸炭異常層の形成状態を示す説明図。
【図２】実施形態例１の歯車を示す説明図。
【図３】実施形態例１における，浸炭処理条件を示す説明図。
【図４】実施形態例２における，再焼戻し前後における浸炭層の硬度を示す説明図。
【図５】実施形態例２における，再焼戻しによる硬度低下状態を示す説明図。
【図６】実施形態例３における，ローラピッチング試験方法を示す説明図。
【図７】実施形態例３における，（ａ）小ローラ，（ｂ）大ローラ，（ｃ）大ローラ端部の形状を示す説明図。
【図８】実施形態例３における，小野式回転曲げ疲労試験の試験片形状を示す説明図。
【符号の説明】
１．．．歯車，
１０．．．母相，
１１．．．浸炭異常層，
１２．．．浸炭層，[0001]
【Technical field】
The present invention relates to a gear widely used in automobiles, construction vehicles, construction equipment, and the like, in particular, a gear excellent in pitching resistance, and a manufacturing method thereof.
[0002]
[Prior art]
In the environment surrounding automobiles, construction vehicles and construction equipment, there is a social demand for energy saving and further performance improvement, and efforts to reduce the weight of the vehicle body and increase the engine output are being promoted. For this reason, the usage environment of gears used in automobiles, construction vehicles and equipment, especially gears used in drive train transmission parts, has become more severe, and has excellent tooth fatigue strength and pitting resistance. Gears equipped are required.
[0003]
Conventional gears used case-hardening steel such as JlS-SCr420 steel, which is chromium steel, or JlS-SCM420 steel, which is chromium molybdenum steel, as the gear steel for producing the gear. These case-hardened steels were formed into gear shapes, and then carburized, quenched, and tempered (hereinafter referred to as carburizing treatment) to form so-called carburized gears.
[0004]
[Problems to be solved]
However, the conventional gear has the following problems.
That is, in recent years, there has been an increasing demand for lighter body and higher engine output required for automobiles, construction vehicles and construction equipment. For this reason, carburized gears that are simply carburized from conventional steel according to conventional standards are becoming unable to satisfy these requirements.
[0005]
On the other hand, many proposals have been made on steel for carburized gears with improved strength, and many proposals have been made regarding manufacturing methods such as shot peening. These proposals have shown dramatic improvements, especially in terms of tooth root strength.
[0006]
However, the improvement of the tooth surface strength is not so great with respect to the improvement of the tooth root strength. For this reason, the gear failure mode changed from root fatigue to tooth surface fatigue, that is, pitching failure. In particular, gears used with high surface pressure and high speed rotation cannot cope with the environment, resulting in a problem of insufficient pitting resistance.
[0007]
For this lack of pitting resistance, it is possible to reduce the amount of oxygen in the steel, to control the form of inclusions by adding trace elements, and to form an abnormal carburization layer by reducing carburizing abnormal layer formation elements. Various methods have been proposed to suppress the erosion or carburizing steel for gears with resistance to temper softening.
[0008]
For example, JP-A-1-52467 discloses an attempt to define the shape of non-metallic inclusions. However, such a proposal requires advanced processing in the steelmaking process, which increases the manufacturing cost and ultimately increases the cost of the steel material, which meets the needs of users who insist on cost reduction. It is gone.
[0009]
Japanese Patent Laid-Open No. 2-85343 discloses a proposal for suppressing the occurrence of a carburized abnormal layer and reducing the tooth surface strength (pitting resistance) by reducing the amount of Si added. There are many other proposals similar to this.
However, according to the detailed investigation and research conducted by the developers, the method of suppressing the carburized abnormal layer does not only provide excellent pitting resistance, but also promotes variations in pitching life of individual gears. I found out.
[0010]
The present invention has been made in view of such a conventional problem, and an object of the present invention is to provide a gear that can be manufactured at low cost and has excellent pitching resistance, and a manufacturing method thereof.
[0011]
[Means for solving problems]
  The invention of claim 1 is a gear obtained by carburizing the case-hardened steel after forming it into a gear shape.
  The above case-hardened steel has a weight ratio of C: 0.10 to 0.30%, Si: 0.50 to 1.50%, Mn: 0.30 to 1.00%, Cr: 0.50. 2.00%, Mo: 0.50% or less, and
  1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%)
And the balance is composed of Fe and inevitable impurities,
  In addition, the carburized gear has a carburized layer having a C concentration of 0.65% or more and a residual austenite amount of 25% or less, and an outer hardened structure has an incompletely hardened structure. Has a carburized abnormal layer,
The maximum depth of the carburizing abnormal layer is 5 to 40 μm, and the area occupied by the carburizing abnormal layer in the cross section from the maximum depth position to the surface is 70% or more,
  And it exists in the gear excellent in pitching resistance characterized by pitching strength being 2600MPa or more.
[0012]
  What should be noted in the present invention is that the case-hardened steel having the above specific composition is used.,Carburized abnormal layer consisting of the above incompletely quenched structure,This is a positive setting.
[0013]
The carburized abnormal layer is a layer composed of an incompletely quenched structure as described above. An incompletely hardened structure is a structure made of troostite or bainite generated during quenching in a series of carburizing processes. This carburized abnormal layer can be easily observed when it is corroded black when it is corroded with a corrosive solution such as nital after mirror finishing of the cross section of the treated product. The carburized abnormal layer is generated as follows.
[0014]
That is, for example, in the case of gas carburizing, a certain amount of oxygen is contained in the carburizing atmosphere. When this oxygen enters from the steel surface, among Si, Cr, Mn, Ni, Mo, etc. contained (solid solution) in the substrate near the grain boundary, Si, Cr, and Mn are crystal grains. Combines with oxygen that has diffused through the field to form oxides. For this reason, the hardenability decreases in the vicinity where the oxide is formed. Therefore, martensite is not generated during quenching, and troostite or bainite is generated. This layer of incompletely quenched structure made of troostite or bainite is a carburized abnormal layer.
[0015]
  This carburizing abnormal layer is provided on the outermost surface of the gear, and the maximum depth is 5 to 40 μm as described above, and the area occupied by the carburizing abnormal layer in the cross section from the maximum depth position to the surface (hereinafter referred to as the occupation) The area ratio is 70% or more.
  As shown in FIG. 1, this abnormal carburizing layer is usually formed with variations in depth. Therefore, in the present invention, the thickness of the carburizing abnormal layer is defined by the maximum depth, and the degree of the unevenness of the depth is defined by the occupation area ratio of the carburizing abnormal layer.
[0016]
When the maximum depth of the carburized abnormal layer is less than 5 μm, there is a problem that the effect of initial conformability described later is not sufficiently exhibited. On the other hand, if it exceeds 40 μm, the amount of wear due to wear of the carburized abnormal layer will be too large, the tooth contact of the gear will deteriorate, the noise during operation will increase, and the stress distribution acting on the tooth surface will be deteriorated. There's a problem.
[0017]
Moreover, when the area occupied by the carburizing abnormal layer between the maximum depth position and the surface is less than 70%, the variation in the depth of the carburizing abnormal layer becomes very large. For this reason, even after the carburizing abnormal layer described later is worn, a large number of soft incompletely quenched structures remain in a wedge shape on the surface of the carburizing layer. Therefore, there is a problem that a crack starting from this tends to occur.
[0018]
On the other hand, the upper limit of the occupied area ratio of the carburized abnormal layer is ideally 100%. In other words, the carburized abnormal layer becomes a uniform layer with less depth unevenness as the occupied area ratio is higher. Therefore, after wear of the carburized abnormal layer, the hardened carburized layer is exposed on the gear surface, and the subsequent pitting resistance can be improved.
[0019]
Next, the carburized layer located below the abnormal carburized layer has a C (carbon) concentration of 0.65% or more and a residual austenite amount of 25% or less as described above.
When the C concentration is less than 0.65%, there is a problem that the hardness of the martensite structure obtained by carburizing and quenching is not so high. However, the upper limit of the C concentration is preferably 1.2% because cementite is generated at the grain boundaries and the fatigue strength and pitting resistance may be reduced.
Further, when the amount of retained austenite in the carburized layer exceeds 25%, there is a problem that the hardness of the carburized layer cannot be sufficiently increased.
[0020]
Next, as the gear material of the present invention, case-hardened steel having the above specific composition is used. Hereinafter, the reasons for limiting each chemical component range will be described.
[0021]
C: 0.10 to 0.30%,
In order to sufficiently satisfy the strength required for the carburized and tempered gear parts, that is, to obtain the internal hardness Hv200 to 500 of the carburized gear parts, 0.10% or more of C is contained. There is a need to. However, if the content exceeds 0.30%, the internal toughness deteriorates, the strength of the gear is reduced, and further the machinability and cold forgeability are deteriorated. did.
[0022]
Si: 0.50 to 1.50%,
During carburizing, Si in the carburized layer reacts with oxygen in the carburizing atmosphere to form oxides. For this reason, the hardenability decreases near the surface layer of the product to be treated, and a so-called carburized abnormal layer is formed. That is, Si is an element that has an important influence on the formation of a carburized abnormal layer, and is an element that increases the temper softening resistance of the martensite structure. In the present invention, it is necessary to contain Si exceeding 0.50% in order to obtain a carburized abnormal layer having a desired form and to improve the temper softening resistance. However, if the content exceeds 1.50%, the above carburized abnormal layer is not obtained, and cold forgeability, machinability, and toughness are lowered, so the upper limit was made 1.50%.
[0023]
Mn: 0.30 to 1.00%,
Mn is an element that has a remarkable effect on improving hardenability, and in order to guarantee the hardness (Hv 200 to 500) necessary to secure the strength to the inside of the gear, Mn of 0.30% or more is required. It is necessary to contain. Furthermore, since Mn is an element that forms an abnormal carburization layer, the amount of addition affects the form of the abnormal carburization layer. For this reason, in order to obtain the carburized abnormal layer necessary for the gear, it must not exceed 1.00%.
[0024]
Cr: 0.50 to 2.00%,
Cr is an element that improves hardenability. After carburizing and quenching, it is necessary to contain 0.30% or more in order to obtain the above internal hardness. On the other hand, if it exceeds 2.00%, the cold forgeability and machinability are remarkably deteriorated, so the upper limit was made 2.00%.
[0025]
Mo: 0.50% or less,
Mo has the effect of improving hardenability and toughness and miniaturizing the crystal grains after carburizing. In addition, there is an effect of suppressing the carburizing abnormal layer, and a carburizing abnormal layer of a desired form can be obtained by using it together with the generation effect of the carburizing abnormal layer possessed by Si. When a large amount of this element is added, not only the desired carburized abnormal layer cannot be obtained, but also the cost is increased, and further, cold forgeability and machinability are deteriorated. Therefore, 0.50% was made the upper limit. In order to exhibit the above effect, the lower limit is preferably 0.10%.
[0026]
Next, the following relational expression for regulating the chemical composition in the case hardening steel will be described.
The relational expression 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%) for regulating the chemical component is a parameter for regulating the tempering resistance of martensite. That is, the tooth surface of the gear is exposed to an environment of 200 to 500 ° C. due to heat generated by friction, and the surface is tempered. As a result, when the decrease in the hardness of the tooth surface is large, that is, when the tempering resistance is low, it causes pitching failure.
[0027]
In the above relational expression, when the value is 1.5 or more, the tempering resistance of martensite is improved, and the decrease in martensite hardness on the tooth surface can be suppressed to Hv100 or less during use of the gear. , Pitting resistance can be improved in terms of tooth surface hardness. The upper limit value of the relational expression is preferably 3.0 for reasons of deterioration of workability (machinability) due to an increase in material hardness and cost increase due to an increase in the amount of alloy elements.
[0028]
Next, the operation of the present invention will be described.
As described above, the gear of the present invention is further provided with the specific amount of carburized abnormal layer on the outer layer of the carburized layer having a specific C concentration and retained austenite. Therefore, the gear of the present invention exhibits excellent initial conformability at the stage of actual use and exhibits excellent pitting resistance.
[0029]
In other words, in the case of carburized gears, it is inevitable that the shape of the tooth surface includes a certain amount of error (strain) if polishing is not performed after the carburizing process. Needless to say, each gear has a slightly different shape for each tooth in a single gear. This error (strain) greatly affects the contact pressure distribution applied to the tooth surface.
As a result, the maximum contact pressure generated on the tooth surface during conventional gear meshing driving is extremely high, and often reaches the limit value of the load capacity. These have a great influence on the pitching life as well as the gear strength, especially the influence of surface pressure.
[0030]
In this regard, the gear of the present invention has the carburizing abnormal layer of the specific thickness on the outer layer of the high-hardness carburizing layer. Therefore, the adverse effect of errors existing on the tooth surface can be greatly mitigated by the running-in operation of the gear incorporated in the device.
That is, the carburized abnormal layer is a soft structure composed of an incompletely quenched structure. Therefore, it has the characteristic that it is easy to wear. This property greatly improves the initial conformability of the gear. Specifically, when the gears are actually engaged and driven, the carburized abnormal layer wears to alleviate the uneven stress distribution generated on the tooth surface, and the tooth surface shape is self-corrected.
[0031]
And the thickness of the carburizing abnormal layer in this invention is 5-40 micrometers. Therefore, this carburized abnormal layer is sufficiently removed by the above-mentioned initial familiarity.
Moreover, in the tooth surface from which the carburized abnormal layer has been removed, the lower carburized layer is exposed on the surface. This carburized layer is composed of a martensite structure having a C concentration of 0.65% or more as described above, and the amount of retained austenite contained is 25% or less, and has a very high hardness.
[0032]
For this reason, in the gear that has been initially adapted, the tooth surface has a shape capable of obtaining a uniform contact state and a uniform high hardness.
Therefore, the gear of the present invention exhibits extremely excellent pitting resistance after the familiar operation.
[0033]
On the other hand, the gear is tempered by being exposed to an environment of 200 to 500 ° C. by frictional heat during use. When the hardness of the tooth surface, that is, the hardness of the carburized layer is reduced by this tempering, the above-mentioned excellent pitting resistance is impaired.
In this respect, the gear of the present invention uses the case-hardened steel having the specific component range as a material. Therefore, it has excellent resistance to softening by tempering. Therefore, the tooth surface strength can be kept high even during operation exposed to high temperatures, and the above-described excellent pitting resistance can be exhibited.
[0034]
In addition, the case-hardened steel in the present invention has not been changed in composition so as to be more expensive than conventional materials. Further, the cost of forming into a gear shape and carburizing treatment can be the same as the conventional one.
Therefore, in the present invention, the gear having the excellent pitching resistance can be obtained at low cost.
[0035]
The carburizing abnormal layer itself can be formed even when a conventional gear steel such as JlS-SCr420 steel or SCM420 steel is used. However, these carburized abnormal layers of conventional steel have a small occupation area ratio (less than 70%). That is, when viewed microscopically, a layer in which an incompletely quenched structure and a martensite structure are mixed is present on the tooth surface.
[0036]
Therefore, in the gears using conventional steel, even if a carburized abnormal layer is provided, not only a good initial running-in state can be obtained, but pitching failure often occurs starting from the remaining incompletely hardened structure. . The martensitic structure of the steel is also inferior in softening resistance.
[0037]
  Next, the claim2In addition to the above composition, the case-hardened steel preferably further contains Al: 0.020 to 0.060% and N: 0.0080 to 0.0200%. Thereby, the strength of the gear can be further improved.
[0038]
Al: 0.020 to 0.060%,
Al combines with N in the steel, and as AlN, has the effect of refining crystal grains after carburizing and quenching to improve toughness. In order to acquire this effect, it is necessary to contain 0.020% or more of Al. However, if it exceeds 0.060%, excessive Al in the steel₂O_ThreeIs generated and the strength decreases, so the upper limit was made 0.060%.
[0039]
N: 0.0080-0.0200%,
As described above, N combines with Al to refine crystal grains as AlN. In order to obtain such an effect, it is necessary to contain 0.0080% or more of N. On the other hand, if the content exceeds 0.0200%, the above effect is saturated and N may gasify during steel making, which may make it difficult to produce steel. Therefore, the upper limit was made 0.0200%.
[0040]
  Claims3In addition to the above-described composition, the case-hardened steel further includes one or two of Ti: 0.20% or less, Nb: 0.20% or less, V: 0, 30% or less. It is preferable to contain more than seeds. Thereby, the strength of the gear can be further increased.
[0041]
Ti: 0.20% or less, V: 0.30% or less, Nb: 0.20% or less,
These elements improve toughness and improve fatigue strength by, for example, refining crystal grains after carburizing. However, even if added in a large amount, these effects are not only saturated, but also coarse precipitates are formed and the strength is lowered. Therefore, the upper limit is set to 0.20% for Ti and 0.30% for V: Nb was 0.20%. In order to sufficiently exhibit the above effects, it is preferable that the lower limit is 0.01% for Ti, 0.03% for V, and 0.01% for Nb.
[0042]
  Next, the claim4In the weight ratio, C: 0.10 to 0.30%, Si: 0.50 to 1.50%, Mn: 0.30 to 1.00%, Cr: 0.50 2.00%, Mo: 0.50% or less, and
  1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%)
Using a case-hardened steel with a composition consisting of Fe and inevitable impurities.
  Forming the case-hardened steel into a gear shape;
  Next, carburizing treatment is performed to form a carburized layer having a C concentration of 0.65% or more and a retained austenite amount of 25% or less, and the carburized layer has an incompletely hardened structure on the outer layer. Forming a layer,
And the maximum depth of the carburizing abnormal layer is 5 to 40 μm, and the area occupied by the carburizing abnormal layer in the cross section from the maximum depth position to the surface is 70% or more,There is a manufacturing method of a gear excellent in pitching resistance characterized by having a pitching strength of 2600 MPa or more..
[0043]
The carburizing process refers to a series of processes including carburizing, quenching, and tempering.
In addition, carburizing can be performed by using various processing methods similar to those in the past. For example, when performing gas carburization, the C concentration of the carburized layer can be easily adjusted by adjusting the carbon potential, temperature, processing time, etc. of the gas to optimum values as is well known.
[0044]
Moreover, after carburizing and adjusting the C concentration, quenching is performed immediately thereafter or after cooling. Quenching may be performed by combining primary quenching and secondary quenching, or may be performed directly. Further, in order to adjust the amount of retained austenite, sub-zero treatment can be performed after quenching.
Further, tempering after quenching is performed at about 130 to 180 ° C. as is usually done.
[0045]
Such a series of carburizing processes is performed on the case-hardened steel having the specific component range formed into a gear shape. Thereby, a gear having the specific carburized layer and the specific carburized abnormal layer can be easily manufactured.
The reason for limiting each numerical value is the same as described above.
[0046]
  Claims5In addition to the above composition, the case-hardened steel preferably further contains Al: 0.020 to 0.060% and N: 0.0080 to 0.0200%.
  In addition, the claims6In addition to the above composition, the case-hardened steel further includes one or two of Ti: 0.20% or less, Nb: 0.20% or less, V: 0, 30% or less. It is preferable to contain the above.
  In these cases, the same effect as described above can be obtained. The reason for the numerical limitation is the same as above.
[0047]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1
A gear excellent in pitching resistance according to an embodiment of the present invention and a manufacturing method thereof will be described with reference to FIGS.
The gear 1 of this example is an example of a general spur gear as shown in FIG. The spur gear is an example, and can be applied to a helical gear, a helical gear, a bevel gear, a screw gear, a worm gear, and other various gears.
In manufacturing this gear 1, first, steel A shown in Table 2 described later was prepared as a case-hardened steel.
[0048]
As shown in Table 2, steel A is a steel in which the contents of C, Si, Mn, Cr, and Mo are regulated within the scope of the present invention, and Al is added by 0.030% and N is added by 0.0120%. In addition, the steel satisfies 1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%).
[0049]
And when producing the gear 1 using A steel, this is first shape | molded by the cutting process to the gear 1 of the shape shown in FIG.
Next, the formed gear 1 is carburized. The carburizing process in this example was performed using a gas carburizing method as shown in FIG.
[0050]
That is, first, the carbon potential (CP) was maintained at 0.8 to 1.2% and carburized for 75 minutes in a gas atmosphere kept at a temperature of 950 ° C., and then slightly C.P. Carbon is diffused into the steel in a gas atmosphere with reduced P. Next, C.I. While maintaining P, the temperature was lowered to 850 ° C., soaked for 30 minutes, and then quenched into 130 ° C. oil. Thereafter, a tempering treatment was performed at a temperature of 160 ° C. for 1 hour.
[0051]
This processing condition can be changed depending on the steel type. For example, the initial carburizing and diffusion temperature can be changed between 930 to 980 ° C., the time between 0.5 and 5 hours, the soaking temperature between 840 and 870 ° C., and the time between 0.5 and 2 hours. .
[0052]
As shown in FIG. 1, the gear 1 obtained by the carburizing process is in a surface state in which a carburized layer 12 is formed on the parent phase 10 and a carburized abnormal layer 11 is further formed thereon.
The carburized layer 12 in this example has a C concentration of 0.70% as a result of analysis by EPMA, and a residual austenite amount of 22% as a result of investigation by an X-ray diffraction method. Further, the hardness of the carburized layer 12 was very high as Hv810.
[0053]
In addition, as shown in FIG. 1, the carburizing abnormal layer 11 has a maximum depth D of 12 μm, and the area occupied by the carburizing abnormal layer 11 in the cross section A from the maximum depth position to the surface is 80%. It was.
[0054]
Next, the operation of the gear 1 of this example will be described.
As described above, the gear 1 of the present example is further provided with the specific amount of carburizing abnormality layer 12 on the outer layer of the high-hardness carburizing layer 12.
Therefore, as shown in FIG. 1, excellent initial conformability is exhibited by engaging and driving the two gears 1.
[0055]
That is, in the gear 1, the carburizing abnormal layer 11 is worn to alleviate the uneven stress distribution generated on the tooth surface 15, and the shape of the tooth surface 15 is self-corrected. And in the tooth surface 15 from which the carburizing abnormal layer 11 was removed by abrasion, the lower carburizing layer 12 is exposed on the surface.
[0056]
Therefore, in the gear 1 of this example, after a certain initial running-in operation, a uniform contact state of the tooth surface 15 is obtained, the stress distribution is uniform, and the contact surface is constituted by a hardened carburized layer. Become so.
As a result, the stress state at the time of contact can be made better and the pitting resistance can be improved as compared with the conventional gears that have taken measures to reduce the carburized abnormal layer as much as possible.
[0057]
Furthermore, the gear 1 of this example uses A steel having the above specific composition as a material. Therefore, there is little decrease in hardness due to frictional heat during gear operation.
Therefore, the gear 1 of the present example has a long life because the pitting resistance does not deteriorate much even after long-term use.
[0058]
Embodiment 2
In this example, in place of the steel A of Example 1, 20 types of test steels shown in Table 1 were prepared to produce gears, and the softening resistance of the carburized layer was quantitatively evaluated.
Production of gears using each test steel was basically performed by the same manufacturing method as in the first embodiment. However, in order to alleviate the effects of differences in hardenability due to differences in components, each gear was subjected to subzero treatment at a temperature of −40 to −70 ° C. for 1 hour after tempering in the carburizing treatment.
[0059]
Further, for the evaluation of softening resistance, after the sub-zero treatment, a re-tempering treatment at a temperature of 250 ° C. for 4 hours was further performed. And the cross-sectional hardness of the carburized layer before and after the re-tempering treatment was measured.
The measurement results are shown in Table 1 and FIG. In addition, Table 1 also shows the following relational expression in the chemical composition of each steel, the value of 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%).
In FIG. 4, the horizontal axis represents the value of the relational expression, the vertical axis represents the hardness (Hv), and the hardness before re-tempering (◯) and after re-tempering (●) is plotted.
[0060]
As can be seen from Table 1 and FIG. 4, the hardness of the carburized layer before re-tempering was as high as around 800 Hv in any of the test steels. On the other hand, after re-tempering, the smaller the value of the above relational formula, the lower the hardness. In particular, as shown in the figure, when the value of the relational expression is less than 1.5, the hardness greatly decreased to a low hardness of less than 700 Hv.
[0061]
In order to clarify the relationship between the hardness decrease and the value of the above relational expression, this is shown in FIG.
In FIG. 5, the horizontal axis represents the value of the relational expression, and the vertical axis represents the hardness decrease value (Hv).
As can be seen from the figure, there is a correlation between the value of the relational expression and the decrease in hardness due to re-tempering. And when the value of a relational expression exceeds 1.5, it turns out that the hardness fall by re-tempering becomes 100 Hv or less.
[0062]
From the above results, the gear of the present invention using the steel in which the value of the above relational expression is regulated to 1.5 or more can suppress softening due to frictional heat when the gear is used, and can improve the pitting resistance of the gear for a long time. It became clear that can be maintained over.
[0063]
[Table 1]

[0064]
Embodiment 3
In this example, as shown in Table 2, in addition to the steel A used in the first embodiment, eight types (9 types in total) of test steels were prepared, and the composition of the case-hardened steel, the depth of the carburized abnormal layer, etc. Of the impact of the pitch on the pitching strength and the like was quantitatively evaluated.
First, Table 2 shows a list of chemical components of the prepared test steels.
[0065]
[Table 2]

[0066]
Next, each test steel was melted, forged and forged, and normalized, then processed into test specimens described later and carburized. As shown in Table 3 to be described later, the carburizing treatment was performed so as to change the maximum depth and occupied area ratio of the carburized abnormal layer while keeping the C concentration of the carburized layer at 0.65% or more. More specifically, for normal gas carburizing, the carburizing method of Embodiment 1 (FIG. 3) is basically used, and the temperature, time, C.I. This was performed by finely adjusting each condition of P. For those that do not generate carburized abnormal layers at all, vacuum gas carburization was performed.
[0067]
Next, as an alternative test for evaluating the tooth surface strength of the gear, a roller pitching test was performed as shown in FIGS.
As shown in FIGS. 6 and 7, the roller pitching test is performed using a small roller 41 having a test portion 411 in the center portion and a disk-shaped large roller 42. The small roller 41 and the large roller 42 are manufactured by performing the same manufacturing process and carburizing process using the same steel type.
[0068]
As shown in FIG. 7A, the small roller 41 has a width W of 28 mm, an outer diameter D at a central portion having an overall length L of 130 mm.₁Is provided with a test portion 411 of 26 mm. The large roller 42 has a thickness T of 18 mm and an outer diameter D as shown in FIG.₂Is a 130 mm disk, and its end 421 has a curved surface with a crowning radius of 300 mm as shown in FIG.
[0069]
Then, as shown in FIG. 6, in a state where the end portion 421 of the large roller 42 set on the shaft 429 and the test portion 411 of the small roller 41 are in contact with each other, the peripheral speed is rotated with a difference. Specifically, the rotation speed of the small roller 41 was 2000 rpm, and the slip ratio (circumferential speed difference) was −40%. The lubrication was performed with ATF (automatic transmission fluid) having an oil temperature of 120 ° C.
[0070]
Then, the rotation is performed with a constant surface pressure applied between the small roller 41 and the large roller 42, and the total number of rotations of the small roller 41 until the occurrence of pitching is obtained. This is repeated by changing the surface pressure. A roller pitching tester manufactured by Komatsu Engineering Co., Ltd. was used.
[0071]
In this example, the small roller 41 is set to 10⁷Surface pressure (10⁷(Endurance surface pressure) was used as the pitching strength (MPa) for evaluation.
The test results are shown in Table 3.
[0072]
As can be seen from Table 3, the pitching strength was as low as 1800 MPa or less for all cases where the maximum depth of the carburized abnormal layer was 0 and over 40 μm (Sample Nos. 1, 6, 12).
Moreover, even if the maximum depth of the carburized abnormal layer is within the range of 5 to 40 μm, if the occupied area ratio of the carburized abnormal layer is less than 70% (sample No. 3, 14), it is also 1800 MPa or less. The value was low. On the contrary, even when the occupied area ratio of the carburized abnormal layer is 70% or more, when the maximum depth of the carburized abnormal layer exceeds 40 μm as described above (Sample No. 6), the value is also as low as 1800 MPa or less. It was.
[0073]
Further, even when the maximum depth and occupied area ratio of the carburized abnormal layer are good, the steel component range is outside the scope of the present invention (Sample No. 13), and the maximum depth and occupied area ratio of the carburized abnormal layer are When the value of the above-mentioned relational expression is less than 1.5 (sample No. 10) even when the steel composition range is within the range of the present invention, the result of the low pitching strength is obtained. It was.
[0074]
Next, an Ono-type rotating bending fatigue test was conducted as an alternative test for estimating the tooth root strength of gears.
The Ono type rotating bending fatigue test was performed according to JIS-Z2274 “Rotating bending fatigue test method of metal material” using a test piece 5 having a shape as shown in FIG. The rotation speed was 3600 rpm.
And in this example, 10⁷Bending stress (10⁷Used for evaluation.
[0075]
The test results are shown in Table 3.
As known from Table 3, sample no. Other than 6, 52Kgf / mm²Thus, good results were obtained. In contrast, sample no. No. 6 had the lowest Ono rotary bending strength. This is thought to be because the maximum depth of the carburized abnormal layer was too deep.
[0076]
[Table 3]

[0077]
From the above test results, it can be seen that the pitching strength can be reliably improved by the composition range of the steel, the provision of the relational expression, and the appropriate formation of the carburized abnormal layer. It can also be seen that within the range in which the pitching strength can be improved, the Ono-type rotary bending strength is also good and the tooth root strength of the gear can be maintained well.
[0078]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a gear that can be manufactured at a low cost and has excellent pitching resistance, and a method for manufacturing the gear.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an explanatory diagram showing a formation state of an abnormal carburization layer in Embodiment 1;
FIG. 2 is an explanatory view showing a gear according to Embodiment 1;
FIG. 3 is an explanatory diagram showing carburizing process conditions in the first embodiment.
4 is an explanatory diagram showing the hardness of a carburized layer before and after re-tempering in Embodiment 2. FIG.
FIG. 5 is an explanatory view showing a hardness reduction state due to re-tempering in Embodiment 2;
6 is an explanatory diagram showing a roller pitching test method in Embodiment 3. FIG.
FIGS. 7A and 7B are explanatory views showing the shapes of (a) a small roller, (b) a large roller, and (c) a large roller end portion in Embodiment 3. FIG.
FIG. 8 is an explanatory view showing a test piece shape of an Ono type rotating bending fatigue test in Example 3;
[Explanation of symbols]
1. . . gear,
10. . . Mother phase,
11. . . Carburized abnormal layer,
12 . . Carburized bed,

Claims (6)

For gears obtained by carburizing after case-hardened steel is formed into a gear shape,
The above case-hardened steel has a weight ratio of C: 0.10 to 0.30%, Si: 0.50 to 1.50%, Mn: 0.30 to 1.00%, Cr: 0.50. 2.00%, Mo: 0.50% or less, and
1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%)
And the balance is composed of Fe and inevitable impurities,
In addition, the carburized gear has a carburized layer having a C concentration of 0.65% or more and a residual austenite amount of 25% or less, and an outer hardened structure has an incompletely hardened structure. Has a carburized abnormal layer,
The maximum depth of the carburizing abnormal layer is 5 to 40 μm, and the area occupied by the carburizing abnormal layer in the cross section from the maximum depth position to the surface is 70% or more,
And the gear excellent in pitching resistance characterized by pitching strength being 2600 Mpa or more. 2. The hardened steel according to claim 1, further comprising Al: 0.020 to 0.060% and N: 0.0080 to 0.0200% in addition to the above composition. A gear with excellent pitching properties. 3. The case-hardened steel according to claim 1 or 2, wherein, in addition to the above composition, Ti: 0.20% or less, Nb: 0.20% or less, V: 0, 30% or less, A gear excellent in pitting resistance characterized by containing two or more kinds. By weight ratio, C: 0.10 to 0.30%, Si: 0.50 to 1.50%, Mn: 0.30 to 1.00%, Cr: 0.50 to 2.00%, Mo : 0.50% or less, and
1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%)
Using a case-hardened steel with a composition consisting of Fe and inevitable impurities.
Forming the case-hardened steel into a gear shape;
Next, carburizing treatment is performed to form a carburized layer having a C concentration of 0.65% or more and a retained austenite amount of 25% or less, and the carburized layer has an incompletely hardened structure on the outer layer. Forming a layer ,
In addition, the maximum depth of the carburized abnormal layer is 5 to 40 μm, and the area occupied by the carburized abnormal layer in the cross section from the maximum depth position to the surface is 70% or more, so that the pitching strength is 2600 MPa or more. A method for producing a gear excellent in pitting resistance, characterized in that In addition to the said composition, the said case hardening steel contains Al: 0.020-0.060%, N: 0.0080-0.0200% in addition to the said composition, The resistance-proof characterized by the above-mentioned. A gear manufacturing method with excellent pitching properties. 6. The case-hardened steel according to claim 4 or 5, in addition to the above composition, Ti: 0.20% or less, Nb: 0.20% or less, V: 0, 30% or less, one or two. The manufacturing method of the gear excellent in pitting resistance characterized by containing the seed | species or more.

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